Method of producing a colored anodic oxide film on aluminum



United States Patent U.S. Cl. 20458 7 Claims ABSTRACT OF THE DISCLOSUREThe subject matter of the specification herein relates to the productionof colored anodic oxide films on aluminum surfaces, which, at the sametime, have excellent resistance to abrasion and weathering. The methodcomprises employing an electrolyte comprising an aqueous solutioncontaining 1.540% by weight of naphthalenedisulfonic acid and 0.13.0% byweight of sulfuric acid at an initial cell voltage of at least 20 volt.

This invention relates to a method of producing an anodic oxide film onaluminum. More particularly, the invention pertains to a method in whichaluminum or an aluminum base alloy (hereinafter referred to as aluminum)is anodized in a novel aqueous acidic solution to form a colored anodicoxide film on the aluminum surface.

A method of producing an anodic oxide film in an electrolyte comprisingan aqueous sulfuric acid solution to form a dense oxide film on thealuminum surface has been most widely carried out on commercial scale.In the above method, the sulfuric acid solution is ordinarily used at atemperature in the vicinity of room temperature, and in order tomaintain said temperature, the electrolyte must be cooled.

Films obtained according to the above method are ordinarily colorless,though they are colored in the case of certain alloys.

Recently a hard anodic oxide film higher in abrasion resistance has cometo be obtained by anodizing aluminum at such a low temperature as aboutC. in an electrolyte comprising an aqueous sulfuric acid solution. Thethus obtained film is not only excellent in abrasion resistance but alsois colored to metallic grey, greyish brown or dark brown. These coloursare attractive for the application of aluminum to architecture and-thelike. However, in order to maintain the electrolyte temperature at about0 C., the expenses of apparatus and operation are necessarily increased.Therefore, the development of more practical process for the productionof such a film has been desired.

An object of the present inventionis to provide a method in whichaluminum is anodized in a novel electrolyte to form a colored anodicoxide film on the aluminum surface.

Another object is to provide a method in which a colored anodic oxidefilm excellent inresistance to abrasion and Weather can be obtained atan electrolyte tempera- 1 3,486,991 Patented Dec. 30, 1969 containing1,5-naphthanedisulfonic acid as the main component is preferable.Aluminum alloys employed in the present invention include every alloywhich may form thick oxide film by anodizing in an electrolytecontaining sulfuric acid.

The colours of the resulting oxide films can be varied by using anelectrolyte within the above component range and varying the kind ofaluminum alloys employed, working conditions of aluminum, and anodizingconditions. The use of the electrolyte of the present invention makes itpossible to obtain, even at a low temperature, such a hard anodic oxidefilm as produced according to the conventional method using an aqueoussulfuric acid solution. Further, at an electrolyte temperature of aboutroom temperature, e.g. 15-30 C., it is also possible to form a colouredanodic oxide film excellent in resistance to abrasion and weather.However, it is not advantageous economically that aluminum alloy isanodized at an electrolyte temperature of 0 C. It is advantageouseconomically that the present electrolyte is used at a temperature of 15to 30 C.

As the concentration of naphthalenedisulfonic acid increases, the cellvoltage becomes lower and the colour of the resulting film becomeslighter. On the other hand, as the concentration of said acid decreases,the cell voltage becomes higher and the colour of the resulting filmgradually becomes deeper from brown to deep brown and black. In case theconcentration of said acid is less than 1.5%, it becomes difiicult toobtain a practical, uniform, colored film. Preferable concentrationrange of naphthalenedisulfonic acid is 520% by weight.

In case the sulfuric acid concentration is less than 0.1% a uniform,colored film is difiicultly obtainable, while in case said concentrationis more than 3.0%, the. colour of the resulting film is getting lighterwith increasing the sulfuric acid concentration and, at last, the filmbecomes colorless, though the cell voltage lowers. Preferableconcentration range of sulfuric acid is 0.31.5% by weight. In case ofelectrolyte comprising an aqueous solution containing 1020% of sulfuricacid and 5-25 of naphthalenedisulfonic acid, the dissolution of theanodic oxide film due to the electrolyte. is inhibited during theelectrolysis to make it possible to use said electrolyte to anodizingtreatment at a high current density. However, the resulting film iscolorless or is merely colored to a slight degree, though the colourdepends on the kind of the alloy employed. In case the presentelectrolyte is used, "the most preferable result may be obtained byanodizing an aluminum at a current density of 1 to 3 a./dm. At anelectrolyte temperature of room temperature, i.e. about 20 C., the cellvoltage becomes 2080 v. in case of the anodizing treatment at a currentdensity ofw1-3 a./dm.

If anodizing is intended to, at the initial voltage of below 20 volt, itis necessary that the anodizing is carried out at lower current densityor at higher electrolyte temperature or the amount of sulfuric acid inthe electrolyte is increased. However in any of these cases a thick,hard and deep coloured objective film cannot be obtained.

At a constant current density, the cell voltage is gradually increasedaccording to the growth of the thickness of the film. There are somecases where, depending on the kind of the aluminum and aluminum alloysemployed, a

' burning phenomenon is observed when the cell voltage has reached above60 v. during the anodizing at the same current density. In such cases,the cell voltage is desirably maintained at below a voltage where theburning phenomenon takes place. Accordingly the anodizing is firstcarried out under a constant current and thereafter under a constantcell voltage in case the burning takes place. The burning phenomenon canalso be prevented by elevating the electrolyte temperature.

As the electric current, either of alternating of direct current isused, and, if necessary, the both may be. superimposed. The anodizingtime is decided according to the current density employed and thedesired thickness of the resulting film, but a colored anodic oxide filmhaving sufa./dm. at the end of the anodizing. The film formed by thistreatment was sealed in the way similar to that of the Example 1. Thefilm colour was brown and the film thickness was about 3511..

Several examples of anodizing conditions and the ficient abrasionresistance can be obtained within an 5 coloured anodic oxide filmsobtained by a method slmilar anodizing time of 3060 minutes. Theobtained film thickto that of the Example 1 or 2 are shown in thefollowing ness 1s usually 15 to 20 per 1 a. hr./dm. table.

Aluminum Electrolyte and Electro- (wt. percent) aluminum Alloycomposition Current Required lyte (remainder; alloy (A. A. densityvoltage temp. Time Film Ex. water) designation) Si Mg Fe Mn Cr Cu Shape(AJdmfl) (v.) 0.) (min) colour 3 Naphthalene- 1100 Nominal impuritiesSheet 2.0 20-30 15-20 60 Yellowish disulfonic silver. acid, 30, nzsoi,1.8. 4 Naphthalene- 6063-1 0.43 0.55 0.17 Extruslonsuu 2.0 20-30 20 Do.

disulfonic acid, 18; HzSO4, 1.2. Naphthalene- 6061-T4 0.63 0.94 0.170.33 0.23 Sheet 3.0 25-60 -20 60 Brown.

disulfonic acid, 15; 112804,]..9. 6 Naphthalene- 6351-1 0.93 0.70 0.300.48 Extrus10ns 1-0 30-45 60 Dark brown.

disulfonic acid, 9; H2804, 0.6. o 6351-T 0.03 0.70 0. 1.5 30-55 20 00Black.

. 6351-Ts 0.03 0. 70 0. 2.0 30-70 20 40 Do. Ourr work- 0. 96 0.67 0. 2.028-70 20 60 Brown.

5- 10 "do d0 0.42 0 62 0.16 0.24 --(10 1.8 30-55 20 60 D0. 11Naphthalene; 6351-T 0. 93 0. 70 0. 39 0.48 -do 1.5 30-56 60 Black.

disulfonic acid, 4.5; HzSO4, 0.3. 12 Naphthalene- Our work- 0.96 0.67 016 0.24 .410 *1.0 35-56 25 60 Brown.

disulfouic T5. acid, 1.5; H2804, 0.1.

*Anodizing treatment was effected at a constant current density untilthe cell voltage had reached 56 v. and thereafter at a constant voltageof 56 v,

In order to illustrate the present invention the following examples aregiven. But the present invention is not intended to be limited to theexamples.

EXAMPLE 1 An aluminum base alloy 6351-T (extrusions) containing 0.93% byweight of silicon, 0.70% by weight of magnesium, 0.39% by weight of ironand 0.48% by weight of manganese was subjected to pretreatments ofdegreasing, etching and desmutting by a usual method and anodized in anaqueous electrolyte containing 9% by weight of naphthalenedisulfonicacid and 0.6% by weight of sulfuric acid at a temperature of 25 C. at acurrent density of 2 a./dm. for 60 minutes under stirring by bubblingair into the electrolyte. The cell voltage increased gradually from v.up to 55 v. at the end of the anodizing. The current used possessed thewave form obtained by full wave rectification of single phase AC.

The film formed by this anodizing was treated with boiling deionizedwater to seal the pores of the film. The colour of the film was blackand the. film thickness was about t.

EXAMPLE 2 Analu-minum base alloy 6063T (extrusions) containing 0.43% byweight of silicon, 0.55% by weight of magnesium and 0.17% by Weight ofiron was subjected to pretreatments similar to that of the Example 1 andanodized in an aqueous electrolyte containing 9% by weight ofnaphthalenedisulfonic acid and 0.6% by weight of sulfuric acid at atemperature of 20 C. with an initial current density of 2.4 a./dm.Within 37 minutes from the. beginning of anodizing the cell voltageincreased gradually from 30 v. to 56 v.

Thereafter the alloy was anodized under a constant voltage of 56 v. for24 minutes. The current density was 1.3

What We claim is:

1. A method of producing a coloured anodic oxide film on aluminum or analuminum alloy, which comprises anodizing aluminum or an aluminum alloyin an electrolyte consisting essentially of a solution containing 1.5-40% by weight of naphthalenedisulfonic acid and 0.1- 3.0% by weight ofsulfuric acid, the remainder by weight being Water, at an initial cellvoltage of at least 20 volt.

2. A method according to claim 1, wherein the anodizing treatment iseffected by use of an electrolyte comprising an aqueous solutioncontaining 520% by weight of naphthalenedisulfonic acid and 03-15% byWeight of sulfuric acid at a current density of 1-3 a./dm. and at anelectrolyte temperature of 15-30 C.

3. A method according to claim 1, wherein the anodizing treatment isfirst effected at a constant current density of 1-3 a./dm. andthereafter at a constant voltage.

4. A methodaccording to claim 2, wherein the anodizing treatment isfirst effected at a constant current density of 1-3 a./dm. andthereafter at a constant voltage.

5. A colored an'odic oxide film on aluminum or an aluminum alloyproduced according to claim 1.

6. A colored anodic oxide film on aluminum or an aluminum alloy producedaccording to claim 2. v

7. A colored anodic oxide film on aluminum or an aluminum alloyproducedaccording to claim 2.

References Cited UNITED STATES PATENTS 2,233,785 3/1941 Korpiun 204--58FOREIGN PATENTS 176 ,639 7/ 193 5 Switzerland.

JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner

